Change default bucket count in hash_set/hash_map.

Change default bucket count in hash_set/hash_map.

Previously these types rehashed to a table of 193 elements
upon construction. But this is non-ideal, first because default
constructors should not allocate unless necessary, and second
because 193 is big and can waste a bunch of memory.

This number had previously been chosen to match GCC's implementation.

llvm-svn: 367605